How it worx:
The Shepard tone:
There are several tones that are pitching up und fading out when they become higher.
At the same time very low tones are fading in.
Easy, eh?
http://upload.wikimedia.org/wikipedia/en/0/0c/Shepard_tone.jpg
Four-Stroke Apparent Motion:
There are 4 pictures of the Rocket.
From pic 1 to 2 the rockets (the background) moves forward.
From pic 2 to 3 the rocket moves backward, but since the contrast is reversed there are no pixels that look like jumping back.
and so on...